[SR-Dev] operators and variables

[Henning Westerholt]

On Thursday 11 December 2008, Andrei Pelinescu-Onciul wrote:
> It would be better to have different operators for strings and for
> integers. Right now we have '+', '==' and '!=' that can be used both for
> strings and integers. The problem with this approach is that it makes
> some optimizations impossible, especially when combined with dynamic
> typed variables (avps or pvars). '+' is especially bad because one
> can tell what type ($v + x) has only at runtime.
>
> I think having perl like operators would help a lot:
> - '.' for string concatenation (instead of reusing '+'), e.g.:
>  $v= "foo" .  "bar"
> - 'eq' instead of == for strings, e.g.: $v eq "bar"
> - 'ne' instead of != for strings, e.g.: $v ne "bar"

Hi Andrei,

first of all, thanks for the work you've done so far for the config scripting.
I understand the problem with the existing operators. But from the user POV i 
doubt that changing this is really worth the effort. This would break 
compatibility with all existing scripts, many documentation out there would 
be obseleted and must be updated and so on.

I don't like the 'eq' and 'ne' that much, for me '==' and '!=' is more 
intuitive as its used in most other programming languages.

Can you estimate the performance benefit that would be possible with this 
optimisations? If its in the range of a few percent, then we should not 
bother about this. The server is really fast enough in my experience, and 
CPUs getting more cores every year.

> We could support them right now in parallel with the old ones and
>  obsolete +, ==  and != for strings in the future (but that means we
>  still cannot optimize '+' in all the cases) or switch right now
> (maybe with some old compat switch which will support old scripts
> style).

I'd prefer to not introduce too much compatibility layers, this makes 
debugging and maintaining both the code and the config script more harder 
then necessary in my experience.

> The question is how much are they used right now. While I think '==' and
> '!=' are used quite often, I'm not sure about '+' for strings (and '+'
> is the most important anyway).

I think the '+' is also commonly used, althought not that much as == and !=, i 
agree. 

> Note: we can still support '==' and '!='  for condition tests not
> involving variables (e.g. method=="INVITE", uri=="sip:foo" a.s.o.).

IMHO having different operators for different type of variables would be 
confusing. The config script language is already quite complicated for 
the "normal" administrator or system developer that don't have the luxury of 
working the whole day with SER/ Kamailio.

> [..]
> In the future it would also be much better to have typed script vars
> (e.g. int $var1). This would help a lot in checking the script for
> correctness. With the dynamic typed vars, one has to run the script to
> find errors. It would also help in optimizing, but not so much if we
> separate the operators, like above.

Having this typed script vars would be ok, but i rather prefer keeping it 
simple and compatible here too, if this brings not a substancial performance 
benefit.

Cheers,

Henning